The present disclosure relates to fusing devices for fusing unfused toner images to a recording medium by inserting and passing the recording medium carrying the unfused toner images into and through a fusing nip formed by a belt and a roller which are pressed against and made contact with each other, and thereby applying heat and pressure to the recording medium, and to image forming apparatuses including such a fusing device.
In conventional image forming apparatuses, toner images formed on an image carrier, such as a photoconductive drum or the like, are transferred to a recording medium. The recording medium carrying the toner images is conveyed toward a fusing device. The fusing device fuses the toner images to the recording medium by applying heat and pressure thereto. Among fusing devices is a belt fusing device. The belt fusing device fuses unfused toner images to a recording medium by inserting and passing the recording medium carrying the unfused toner image into and through a fusing nip. The fusing nip is formed by an endless fusing belt heated and a pressure roller. The fusing belt is pressed against and made contact with the pressure roller.
In the belt fusing device, a pressing member is provided inside the fusing belt, and the pressure roller is pressed against the pressing member from the outside of the fusing belt. Also, in the belt fusing device, the fusing belt is driven to rotate while sliding on the pressing member with the inner surface of the fusing belt being in contact with the pressing member. Also, in the belt fusing device, the surface of the pressing member on which the fusing belt slides has a flat or arc-shaped surface to provide a wide nip width, whereby fusing performance and recording medium releasability are improved.
However, if the pressing member has the arc-shaped sliding surface, then when unfused toner images are fused to a recording medium, such as an envelope or the like, the front and back sides of the envelope have different conveyance speeds at the fusing nip, so that the front and back sides of the envelope are wrinkled, or the flap of the envelope is not correctly placed at a predetermined position (flap misalignment).
There is a known conventional technique of preventing or reducing the formation of wrinkles and the like on an envelope during the fusing process. For example, some fusing devices include a pressing member including a soft pad of an elastic soft material and a hard pad of a hard material, and a switching mechanism which can switch the fusing device between a normal mode in which fusing is performed on plain paper, such as A4 paper and the like, and an envelope mode. In the normal mode, the soft and hard pads are pressed against and made contact with the fusing belt, whereby a fusing nip is formed. On the other hand, in the envelope mode, the hard pad is separated by the switching mechanism, so that only the soft pad is pressed against and made contact with the fusing belt, whereby a fusing nip is formed. As a result, the nip pressure is reduced, and therefore, wrinkles and flap misalignment of the envelope are reduced.